Evolution of Superconductivity with Sr-Deficiency in Antiperovskite Oxide Sr3−xSnO

Abstract

Bulk superconductivity was recently reported in the antiperovskite oxide Sr3−xSnO, with a possibility of hosting topological superconductivity. We investigated the evolution of superconducting properties such as the transition temperature Tc and the size of the diamagnetic signal, as well as normal-state electronic and crystalline properties, with varying the nominal Sr deficiency x0. Polycrystalline Sr3−xSnO was obtained up to x0 = 0:6 with a small amount of SrO impurities. The amount of impurities increases for x0 > 0.6, suggesting phase instability for high deficiency. Mössbauer spectroscopy reveals an unusual Sn4− ionic state in both stoichiometric and deficient samples. By objectively analyzing superconducting diamagnetism data obtained from a large number of samples, we conclude that the optimal x0 lies in the range 0.5 < x0 < 0.6. In all superconducting samples, two superconducting phases appear concurrently that originate from Sr3−xSnO but with varying intensities. These results clarify the Sr deficiency dependence of the normal and superconducting properties of the antiperovskite oxide Sr3−xSnO will ignite future work on this class of materials.

Negative ionic states of tin in the oxide superconductor Sr3−xSnO revealed by Mössbauer spectroscopy

“…We comment that the phase splitting into the nearly stoichiometric and deficient compounds has been suggested in the previous study of magnetization [16] and indeed observed with NMR [15]. Since NMR probes properties of the conduction electrons though it measures the Zeeman energy of the Sn nucleus, the splitting in the NMR spectrum suggests that one sample consists of two materials ("phases") with distinct stoichiometries as shown in Fig.…”

“…Due to the recent predictions of bulk Dirac cones [3,4] and topological nature [5] in the vicinity of the Fermi energy, antiperovskite oxides are attracting a lot of attention [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. After the initial theoretical predictions, the chemical and physical characters of antiperovskite oxides have been extensively investigated [6].…”

“…Band structure calculations [11] and NMR experiments [15] suggest heavy hole doping to Sn due to Sr deficiency. Indeed, Mössbauer spectra at room temperature revealed additional ionic state of Sn having less electrons than Sn 4 -, and its fraction increases with Sr deficiency [16]. However, the microscopic relation between the Sn states and Sr deficiency was not understood.…”

Unusual valence state in the antiperovskites Sr3SnO and Sr3PbO revealed by x-ray photoelectron spectroscopy

“…Experimentally, angle-resolved photoemission spectroscopy [11], magnetotransport [12,13] and nuclear magnetic resonance [14] measurements have probed the possible 3D Dirac nature of the electrons in these compounds. Experiments have also revealed signatures of ferromagnetism arising from oxygen vacancies [15,16], high thermoelectric performance [17], superconductivity arising from Sr vacancies [18,19] and weak antilocalization due to spin-orbital entanglement [20]. The rich electronic properties of the antiperovskites take as their fundamental origin the unusual valence state of B (= Sn, Pb).…”

Normal-state properties of the antiperovskite oxide Sr3−xSnO revealed by Sn119 -NMR

“…When A is a group-2 element and B is tin or lead, the valence of the B ion becomes 4− to satisfy the charge neutrality condition [6]. Indeed, Sn 4− state was experimentally confirmed in Sr 3−x SnO by Mössbauer measurements [7]; such a negative-ionic state of a metallic element is quite unusual.…”